Waves, Light & Sound
Light: Reflection, Refraction, Lenses & Colour
1st Year · 2nd Year · 3rd Year
- ✓By the end of this lesson students will be able to describe reflection and refraction of light.
- ✓By the end of this lesson students will be able to identify and explain the action of converging and diverging lenses.
- ✓By the end of this lesson students will be able to explain the dispersion of white light into the visible spectrum.
- ✓By the end of this lesson students will be able to identify the primary colours of light and explain how objects appear to have colour.
Key concepts
Reflection is the bouncing back of light when it strikes a surface. When light hits a smooth, shiny surface like a mirror, it reflects in a regular way (specular reflection). When it hits a rough surface, it reflects in many different directions (diffuse reflection).
The Law of Reflection states that the angle of incidence is equal to the angle of reflection. The incident ray, the reflected ray, and the normal (an imaginary line perpendicular to the surface at the point of incidence) all lie in the same plane. The angle of incidence is the angle between the incident ray and the normal. The angle of reflection is the angle between the reflected ray and the normal.
Refraction is the bending of light as it passes from one transparent medium into another, for example, from air into water or glass. This bending occurs because light changes speed as it moves from one medium to another. If light enters a denser medium (e.g., from air to glass) at an angle, it bends towards the normal. If it enters a less dense medium (e.g., from glass to air), it bends away from the normal.
A converging lens is thicker in the middle and thinner at the edges. It causes parallel rays of light to bend inwards and meet at a single point called the principal focus (F). Converging lenses are used in magnifying glasses, cameras, and to correct long-sightedness.
A diverging lens is thinner in the middle and thicker at the edges. It causes parallel rays of light to spread out or diverge. The rays appear to come from a single point called the principal focus (F) on the same side as the incident light. Diverging lenses are used to correct short-sightedness.
Dispersion is the splitting of white light into its constituent colours when it passes through a prism or water droplets. This happens because different colours of light travel at slightly different speeds in the medium, causing them to refract by different amounts. Red light is refracted least, and violet light is refracted most.
The visible spectrum is the range of colours that make up white light, which our eyes can detect. These colours are always in the same order: Red, Orange, Yellow, Green, Blue, Indigo, Violet (ROYGBIV). This is the spectrum seen in a rainbow.
The primary colours of light are Red, Green, and Blue (RGB). These three colours can be mixed in different proportions to produce all other colours of light. When all three primary colours of light are mixed together in equal amounts, they produce white light.
The secondary colours of light are formed by mixing two primary colours of light. These are: Yellow (Red + Green), Cyan (Green + Blue), and Magenta (Red + Blue).
The colour of an opaque object depends on the colours of light it reflects and the colours it absorbs. For example, a red apple appears red because it reflects red light and absorbs all other colours. If an object reflects all colours, it appears white. If it absorbs all colours, it appears black.
Key facts to remember
- 1Light travels in straight lines.
- 2The Law of Reflection states that the angle of incidence equals the angle of reflection.
- 3Refraction is the bending of light as it passes from one medium to another due to a change in speed.
- 4Converging (convex) lenses bring parallel light rays to a focus.
- 5Diverging (concave) lenses spread parallel light rays out.
- 6White light can be dispersed into the visible spectrum (ROYGBIV) using a prism.
- 7The primary colours of light are Red, Green, and Blue (RGB).
- 8The colour of an opaque object is determined by the colours of light it reflects.
Worked examples
Example 1
A ray of light strikes a plane mirror at an angle of incidence of 35°. What is the angle of reflection?
Answer
The angle of reflection is 35°.
Always measure angles from the normal, not from the mirror surface.
Example 2
Describe what happens to a ray of light as it passes from air into a block of glass at an angle.
Answer
As the ray of light passes from air into the block of glass, it changes speed and bends towards the normal. This is because glass is optically denser than air.
Example 3
If a green object is viewed under red light, what colour will it appear? Explain your answer.
Answer
A green object viewed under red light will appear black. This is because the green object absorbs red light and only reflects green light. Since there is no green light present to be reflected, no light is reflected to our eyes, making the object appear black.
Think about what colours are available to be reflected by the object.
Common mistakes
- ✗Confusing the angle of incidence/reflection with the angle between the ray and the mirror surface.
- ✗Mixing up the effects of converging and diverging lenses.
- ✗Confusing the primary colours of light (RGB) with the primary colours of pigment (Red, Yellow, Blue).
- ✗Incorrectly explaining why an object appears a certain colour when viewed under different coloured light.
Exam tips
- ★Always draw a normal (a line perpendicular to the surface) when drawing diagrams for reflection and refraction.
- ★Remember the order of colours in the visible spectrum: ROYGBIV.
- ★Practice drawing simple ray diagrams for lenses to understand their effect on light.
- ★For colour questions, clearly state what colours are absorbed and what colours are reflected by the object.
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